populating unexpected value (question mark symbol ?) in string value

[sunil maddheshi]

Hi everyone

We have a string field highlighted in screenshot where we are populating some value based on calculation, but for few records it is showing question mark(?) symbol in that field. What could be the reason..Please see screenshot

[Dr Atul - LNG]

It there any help text/ghost text/example text is there?

[sunil maddheshi]

Hi Atul

i can not see any help text in dictionary of that field